The big picture: circulation and gas exchange



  • Direct exchange between every cell and the environment is not possible in all animals
    • special structures for exchange & transport needed


  • Respiratory systems:


  • Circulatory systems:


  • Internal transport and gas exchange functionally related

Molecule trading



  • Small molecules can move between cells and their surroundings by diffusion


  • Diffusion is only efficient over small distances


  • Molecular trading in animals occurs for every cell!
    • gain O2 and nutrients
    • release CO2 and waste products


  • Simple body plans:
  • Complex body plans:

Animals with gastrovascular cavities



  • Many animals have lots of cells in contact with environment
    • hydras, jellyfish, flatworms


  • Central gastrovascular cavity functions to distribute substances
    • opening at one end connects cavity to water
    • allows fluid to bath inner and outer tissues
    • diffusion distance kept short


  • Gastrovascular cavity can be highly branched
    • extends into tentacles of cnidarians
    • covers high surface area of flatworms

Animals with circulatory systems



  • Circulatory systems have 3 components


  • Fluid transport, via vessels, connects cytoplasm of cells to organs for gas exchange


  • Mammals → O2 diffuses over 2 cell layers in lungs to reach blood
    • molecule trading works like a circuit

Open and closed circulatory systems (hemolymph vs blood)


Vertebrate Circulatory systems



  • Cardiovascular system: blood → heart → vessels
    • length of vessels can be staggering


  • 3 main types of vessels
  1. Arteries:
  2. Veins:
  3. Capillaries:


  • Hearts of vertebrates have 2+ muscular chambers
    • atria:
    • ventricles:


  • Number of chambers related to form and function

Single vs Double circulation


Evolutionary variation: 3 chambered heart




  • Amphibians and reptile hearts have 1 ventricle
    • still use double circulation


  • Do not always fill their lungs
    • pass long periods without gas exchange
    • or use another tissue (skin)


  • Example: frogs use skin when underwater
  • Example: crocodiles shunt blood from lungs when underwater

Mammalian Circulation (1st circuit)




  • The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle


  • Right ventricle pumps blood to the lungs via arteries


  • Blood flows through capillaries into lungs and loads O2 and unloads CO2


  • O2 rich blood returns from lungs via veins to the left atrium

Mammalian Circulation (2nd circuit)



  • Blood flows to left ventricle and pumped to tissues via arteries into capillaries
    • First branches supply the heart muscle
    • Further branches lead to organs and hind limbs


  • O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood


  • Capillaries rejoin conveying blood to veins
    • O2 poor blood from the head, neck, and forelimbs is emptied into right atrium

Blood flow




  • Physical laws govern movement of fluids through pipes
    • affect blood flow and pressure


  • Blood flows from areas of higher pressure to low
    • narrow diameter capillaries decrease pressure


  • Blood flow is slowest in the capillaries
    • over large cross-sectional area
    • necessary for gas exchange

Gas exchange over respiratory surfaces





  • Gas exchange = uptake of O2 and release of CO2
    • O2 is abundant in the air (21%)
    • fairly easily to ‘breath’


  • Water is more demanding for gas exchange
    • dissolved O2 is variable but always less than air


  • Efficient surfaces for gas exchange have evolved

Respiratory Surfaces



  • Differs across animal phylogeny
    • sponges, cnidaria, flatworm → cells directly
    • earthworms, amphibians → skin


  • Other animals lack enough surface area
    • to much volume to supply with O2


  • Respiratory organs evolve
    • Gills → aquatic animals
    • Trachea → insects
    • Lungs → reptiles and mammals

Gills


Trachea in insects


Lungs in mammals, reptiles and birds


Coordination of circulation and gas exchange




  • Most animals need to exchange large amounts of O2 and CO2
    • to meet metabolic needs


  • During inhalation, fresh air fills in lung
    • molecule trading occurs via diffusion


  • Blood is then pumped through circuit
    • O2 carried in pigments (metal bound to protein)